Four American citizens have died in an apparent suicide bomb attack at the infamous Bagram Airfield in Afghanistan. The airfield, which has been serving as a NATO mission headquarters also happens to be the largest U.S. Airfield in Afghanistan.

According to U.S. Defense Secretary Ash Carter, the suicide attack happened early Saturday morning. He confirmed that two of the U.S. citizens were service members, while the other two were contractors. Carter added that he was “deeply saddened” by the news of the attack and the subsequent deaths. A Polish citizen too has died in the attacks, according to a CNN report.

The attack has also resulted in several injuries with reports of more than 16 people injured. Initial reports say that the attack happened on a sidewalk where people were preparing for a morning run. The suicide bomber was dressed up like a local laborer who wore a bomb laden vest underneath his clothes.

Carter released a statement hours after the attack which read as follows.

“I am deeply saddened to learn that an explosion early this morning at Bagram Airfield in Afghanistan has resulted in U.S. casualties. An apparent suicide bomber has taken the lives of two U.S. service members and two U.S. contractors working on the base. The explosion wounded 16 other U.S. service members and one Polish soldier participating in our NATO mission. I want to express my sincere condolences to the families of the fallen, and I want to reassure the loved ones of those injured that they are getting the best possible care. Force protection is always a top priority for us in Afghanistan, and we will investigate this tragedy to determine any steps we can take to improve it.”

Meanwhile, before the investigations into the attack started, the Taliban has claimed responsibility for the attack. A Talibani Twitter handle praised the “strong attack” and warned of more such attacks.

Independent confirmation also came from Taliban spokesman Zabihullah Mujahid who issued a statement confirming their individual. In the statement, he added that the suicide bomber had targeted “a sports ground where more than 100 military officers, important people and soldiers were busy exercising.”

He also confirmed that this attack was under planning for several months, before being executed today.

Following the attack, the air force base was under lockdown for several hours. Security was tightened across all other U.S. installations and bases across Afghanistan to prevent any other such attack. These measures included the closure of the U.S. Embassy in Kabul tomorrow – on Sunday.

A senior NATO official had earlier revealed that the suicide attack happened at around 5:30 a.m., local time and that 14 people were injured in the attack. NATO Secretary General Jens Stoltenberg tweeted about the incident on twitter.

My thoughts go to the loved ones of those killed at #Bagram Airfield & the wounded. #NATO stands w/ #Afghanistan in the fight against terror

“NATO remains committed to our mission to train and support the Afghan forces. We all stand united in the fight against terrorism.

Condemnation also came from the president of Afghanistan Mohammad Ashraf Ghani who termed the attack a “cowardly act by terrorists who have no regard for Islam, human life or the future of Afghanistan.”

He went on to add that all measures would be taken to destroy the enemies of Afghanistan and vowed to bring those responsible for the attacks to justice.

Bagram airfield has been the target of several terrorist attack in the past as well. It is located 30 miles north of Kabul near to the ancient city of Bagram. Officials are now investigating how the attacker managed to enter the highly secure air base. According to estimates, more than 14,000 people work at the Bagram airfield with almost 40 percent of them being contractors.